CAIRO - Egyptian Islamists went on the counterattack on Thursday after a bloody crackdown on supporters of ousted president Mohamed Morsi, raising fears of a wave of violence in the regional power.

The clashes killed hundreds and drew international censure.

The country awakened to an unusual calm after an overnight curfew imposed by the army-backed government, but Morsi supporters vowed to rally again to demand his reinstatement and violence erupted as the day progressed.

Islamists attacked police facilities in the Sinai and the central city of Assiut, killing two policemen, and also torched the headquarters of the provincial government in Giza, near the capital.

Militants in Egypt's Sinai peninsula on Thursday killed seven soldiers in an attack on a checkpoint, security officials said.

In Alexandria, hundreds of Morsi supporters cut the road on the corniche, chanting for their deposed president, state media reported.

In Beni Sueif province, they took to the streets to denounce the police and army crackdown Wednesday on two protest camps in Cairo that sparked nationwide clashes in which at least 525 people died.

The Muslim Brotherhood, from which Morsi hails, said a march was planned later Thursday from the Al-Iman mosque in the capital "to protest the death of their relatives".

Spokesman Gehad al-Haddad said demonstrations would continue.

"We will always be non-violent and peaceful. We remain strong, defiant and resolved," he tweeted. "We will push forward until we bring down this military coup."

Morsi loyalists have insisted their demonstrations are exclusively peaceful, but an AFP reporter saw several protesters carrying weapons at Rabaa al-Adawiya, one of the camps, on Wednesday.

In Cairo, trucks cleared debris from the charred sites there and at the Nahda Square protest camp, both occupied for weeks by Morsi loyalists after he was ousted by the military on July 3.

As the death toll from the carnage soared, condemnation of the assaults poured in, with France warning of the threat of "civil war" and Turkey demanding UN action.

President Barack Obama said Thursday the United States has cancelled military exercises with Egypt to protest the killing of hundreds of protesters.

And Wednesday may have just been the start of a vicious cycle of violence, analysts warned.

Numerous churches were attacked as police broke up the protests, with Christian activists accusing Morsi loyalists of waging "a war of retaliation against Copts in Egypt".

Wednesday's violence was Egypt's worst in decades, exceeding even that seen during the 18-day uprising that ousted president Hosni Mubarak in 2011.

While the health ministry put the toll at 525, the Brotherhood spoke of 2,200 dead overall and more than 10,000 wounded.

The killing prompted interim vice president and Nobel laureate Mohamed ElBaradei to resign, saying he was troubled over the loss of life, "particularly as I believe it could have been avoided".

Despite the condemnation from abroad, Egyptian interim prime minister Hazem al-Beblawi praised the police for their "self-restraint" and said the government remained committed to an army-drafted roadmap calling for elections in 2014.

Beblawi justified the use of force saying Morsi loyalists had been sowing chaos around the country, "terrorising citizens, attacking public and private property."

"The state had to intervene to restore security and peace for Egyptians," Beblawi said, adding that a state of emergency declared by the president was necessary under the circumstances.

He urged all sides to work for the next phase in preparation for fresh elections in 2014.

"This government is committed to the roadmap, to having a consensus constitution that protects all rights... based on social justice."

Wednesday's violence was a dramatic turn of events for the Brotherhood, which just over a year ago celebrated Morsi's victory as Egypt's first elected president.

His single year in power, marred by political turmoil, deadly clashes and a crippling economic crisis, turned many against the Islamist movement, with millions taking to the streets on June 30 to call for his removal.

*This article has been updated with comments from analysts, new detail of attacks and a warning from police.